1. Field of the Invention
The present invention relates generally to the electronic shift control of automatic transmissions in accordance with signals determined by the actuation of a remote manually operable range selector lever and more particularly to the detection of an abnormal shift control in which the transmission is not shifted as instructed by the range selector lever.
2. Description of the Prior Art
An example of a prior art automatic transmission shift control device is shown in Japanese Utility Model Provisional Publication No. 62-41954. With the prior shift control device, an inhibitor switch detects the shift position of a manual lever of an automatic transmission and supplies a signal representative thereof to a shift controller for feedback shift control.
The prior art shift control device performes a shift control as shown in the flow chart of FIG. 5. That is, at step "S1" it is judged whether a shift instruction is made. When a shift insrtruction is actually made, the control proceeds to step "S2" where it is judged whether the shift instruction is proper in view of vehicle conditions such as a vehicle speed, an engine throttle opening, etc. When the shift instruction is proper in view of the vehicle conditions, the control proceeds to step "S3" where an actuation signal such as a drive pulse is supplied to a stepping motor or the like actuator. In this connection, the amount of drive by the actuator is determined depending upon the shift ranges between which a shift of the transmission is to be made. For example, the amounts and directions of drive steps are determined in accordance with the Table of FIG. 6 and stored in the shift controller. That is, where the transmission is to be shifted from the parking range "P" to the drive range "D", the actuator drives the manual lever of the transmission by S4 step in the normal direction. On the other hand, in the case that the transmission is to be shifted from the drive range "D" to the parking range "P", the actuator drives the manulal lever by S'4 step (i.e., by S4 step in the reverse direction). Then, at step "S4" it is judged whether it has passed a predeterminded time necessary for completing the shift. For example, in the case that the transmission is to be shifted from the parking range "P" to the drive range "D", it is judged at step "S4" whether it has passed a time necessary for completing the drive of the S4 step. After a predetermined time has passed, it is judged at step "S5" whether the current shift range indicated by the inhibitor switch is coincident with the shift range indicated by the shift instruction signal. In the case that the shift ranges indicated by the current shift range signal and the shift instruction signal are coincident with each other, the control is returned back to step "S1" to make ready for a next shift instruction by the driver. In the case that the shift ranges indicated by the current shift range signal and the shift instruction signal are not coincident with each other, the control proceeds to step "S10" where a buzzer is activated to inform the driver of the abnormal condition of the shift control device and at the same time the shift control device is stopped and disabled to make a further range shift.
In the prior art shift control device, whether the control device is in the normal condition is judged based upon whether the shift ranges indicated by the current shift range signal and the shift instruction signal are coincident with each other. Due to this, if the inhibitor switch happens to have a temporary contact defect and is thereby disabled to detect the current shift range properly, it is judged that the shift control device is in an abnormal condition and therefore the shift control device is stopped and disabled to make a further range shift. Accordingly, even if the inhibitor switch is defective only temporarily and therefore recovers its properly operable state soon after, the shift control device is held inoperable.